150sx vs yz250 for 2nd bike?

Are you planning on doing the maintenance on them yourself ? 
being heavy will definitely take its toll on the 150. I had one too and loved it. But the pistons wear quickly and if not maintained will cause more problems. 
something to consider 

I would buy the Yamaha just for the suspension 

Why not a 250sx? At least parts between the 350 some of it could be shared. If I had to choose and I have a 150sx. Your size I'd go 250. 

I would choose the 150. A small bore 2-stroke is the perfect second bike.

Might want to see what the parts are costing these days, that might help your decision. 

Riding a small bore correctly is unbelievably rewarding! 
you have to do everything correctly to make it work if your heavy like me, but when you do. Big grins and the best noise ever! Not fun for lazy short shifters for sure tho. No offense to lazy short shifters as I am one sometimes on big bikes 

I'm vet b and always did pretty well on a yz 250.  I've ridden 150 and 350 a few times I feel like a yz is closer feeling to the 350 and a 150 was light and really fun. Just my opinion I'm fairly light 150 pounds so I don't ride 450s at all.

If you can get a low hours carb sx150, go for it... even at 250lbs, my 150 would pull 3rd through a lot of corners. Two ring pistons go for a really long time also,  much more than 15 hours.

Yz250 is another good option,  since they are easy to dial and have lots of parts.

For a second bike, it needs to be easy, not sure if would go wirh a rfi 125,150 just yet.

YZ125.

It's like a KTM 150 except it works properly. 

205lbs and have put 40 moto hours on my 23 Yz 250. Such a phenomenal bike. 

I have a TM 144. The engine is a purpose built 144 - not a big bore 125 - so it has a very proper powerband that revs out like a 125 but is more forgiving throughout. The build quality is impeccable. And the suspension works wonderfully just with heavier springs (KYB SSS forks, a very good in house TM shock). It's hand built and the linkage even comes properly greased. I have no hesitation in saying it's better than the KTM, but it's pricey and tough to resell. I've had no trouble getting parts. I got 50 hours out of a piston, but even though I'm heavy, I'm really slow (I am in the early stages of Parkinson's so there's not many days when I can really push the bike - though I do have a 21 year old son who occasionally rides it).  

Mine is a 21 but my buddy has a 23 (redesigned) and he says it feels just like mine but corners a bit better (which could come down to setup). Full disclosure, my buddy is a TM dealer, but if he thinks something is crap he'll tell me. If you don't mind spending the $, you wouldn't regret it. 

There’s a few Vets that can ride the piss out of a small bore… if OP is one of those guys cool, if not go with 250cc, never have to worry about clearing a jump out of a corner.  I’ve had several YZ250’s and they are great bikes, a handful, but rewarding.

The Boyesen Rad valve fills in any power gaps. 

I always kept 125/150’s for a second bike but finally realized I’m too heavy. I keep a yz250 now and it’s so much fun to hop on when I want to change it up from the 450. 

I would buy one just to avoid TBI.   And the suspension is a major bonus.

I'm being downvoted above you, but we know better 

I have owned 2013 KTM 150SX for over ten years now and it's the most fun to ride bike that I have ever owned. For the pure fun factor I would definitely choose the 150 over 250 which can quite of a handful when you are getting tired.

We're nearly the same age, weight, and skill and I vote YZ250.....oh which I have one myself. First off, super reliable, easy to work on, and they're endless possibilities to mod it up to fit your liking. 

Couple suggestions. Get a gytr weighted flywheel, at minimum swap out the 50 power jet for a 40 (jet the bike according to your specific area of course), and get stiffer springs (maybe .46 fork and 5.4/5.6 shock).

I almost got a 125/150 instead of the 250 but it's just too slow for some of the tracks in my area. My YZ does everything and more than I'll ever need....and gives me the dose of 2 smoker fun that we all love. 

I see you're in Australia so I imagine it's pretty warm in the summers...not sure of the humidity though. This is my baseline jetting for my YZ250 for central/south Texas (pump gas works with it but mix 50/50 with VP vpr and it's solid). Just remember to change out the power jet to a 40 first thing then mess with the rest. 

Main: 178 / 175 summer

Pilot: 48 / 45 summer (stock is 50)

Needle: N3EJ (stock is N3EW)

Clip: 3rd / 2nd summer

Air screw: start with 1.25 and adjust to the day. 

Yz250 with a flywheel weight (like 9oz maybe) 👍

As an owner of CR125 and YZ250, both stock (only the YZ has a gytr heavier flywheel), I'll tell you that both are a great option as a second bike.
For me, smaller the displacement,  more fun, but also more maintenance, top ends, etc...
It all depends on the hours you are going to do at that second dirtbike:

Few hours, go to the sx150
Many hours, go to the yz250

Cheers.

I’m 190-200 te150. Single track mostly. Bike is a riot and only an issue on long loose hills. But even that’s not bad just notice able with the on off power

You will not regret buying a yz250 either, they are great do it all bikes with no downside. Including no e start

Get a carbed 150 and throw the money u save on some Mods and proper suspension, it Will be more reliablethen the 25 be faster,lighter and also handle way better. 

I Just got one and its the most fun bike I have ever owned. 

Saying that i also owned yz250s multiple yz125s and my last bike was a 2022 sx250